The present inventive subject matter relates to the apparatus and methods for electrofishing in general and for a modifiable portable (“backpack”) electrofisher in particular.
Electrofishing
The protection and preservation of natural resources includes the management of fish and game. Fish move about lakes, rivers, streams and reservoirs for a variety of reasons, including migration, spawning, and searching for food. Therefore fishery biologists have relied on a number of tools to determine the number of fish in a lake or stream. These techniques include using nets, hook and line fishing, sonar imaging, snorkel counting, and/or electrofishing. Electrofishing is a popular technique in aiding the fisheries biologist in capturing fish for study.
There are many uses for electrofishing which are carried out to the extent the equipment available permits and without inducing an unacceptable mortality rate. Electrofishing permits the capture and removal of fish population from one locality to another. It permits surveys or population estimates to determine the type and number of fish present and their size range. Such estimates may also uncover natural fluctuations in population and assess the impact of channelization. Electrofishing permits guiding the movement of fish such as keeping predators away from freshly planted fry, keeping fish away from electric power plant water intakes, keeping migrating fish away from specific areas en route and trapping fish (such as the sea lampreys in the Great Lakes). Biological sampling may be accomplished through electrofishing to collect brood stock, determine species composition, tabulate size and age characteristics. Food habits can be determined by collecting fish at feeding areas. Tagging and marking studies are readily carried out by acquiring fish by electrofishing, provided that the stresses of handling and shocking do not injure or kill too large a percentage. The prospect of injury and death is of particular concern when one is dealing with endangered species of fish.
Apparatus and techniques of electrofishing are the subject of U.S. Pat. Nos. 5,445,111; 5,327,854; 4,672,967; 4,713,315; 5,111,379; 5,233,782; 5,270,912; 5,305,711; 5,311,694; 5,327,668; 5,341,764; 5,551,377; and 6,978,734 which are incorporated herein by reference. Furthermore, techniques of electrofishing and the results of electrofishing have been described in numerous journals.
Electrofishing and Fish Physiology
The fish's body acts as a “voltage divider” when swimming through fresh water, and the gradient of an electrical field in the body of a fish will typically be less than the voltage gradient in the same space filled by fresh water. That is, the voltage gradient is altered in a region proximate a fish in the zone of an electrofisher. The voltage gradient across the body of a fish will be roughly proportional to the voltage gradient in the same region of fresh water when no fish are present. Accordingly, if the voltage gradient in a region of water is doubled, the voltage gradient across the fish (and the electrical current through the fish) will also double.
The effectiveness of an electric fish barrier on a particular fish, therefore, depends on the voltage field gradient produced by the electric fish barrier. The maximum transfer of energy from water to a fish occurs when the fish's electrical conductivity matches the electrical conductivity of the surrounding water.
The physiological reaction in the fish is dependent on the voltage gradients across the body of the fish. If a voltage gradient in a region of water is too weak, the fish will not feel appreciable discomfort, and will travel undaunted by the electric fish barrier. An “annoying region” will cause a fish to turn around and travel the preferred route.
The current passing through a fish generated by an electrofisher depends on a variety of factors such as the conductivity of the water at both ends of the fish, the total resistance in a conductive path of water, and the size and species of a fish.
During electrofishing with pulsed DC electric current, a fish will enter into a number of physiological states, depending upon the field strength or density in which it finds itself and upon the frequency, shape and width of the pulses. Five of the prominent physiological states are: (1) frightening the fish; (2) electrotaxis, the involuntary exercise of swimming muscles to draw the fish toward the source of electric current; (3) narcosis, when the fish muscles go limp and the fish rolls on its side which facilitates netting and acquisition of the fish; (4) is tetanus which is an involuntary contraction of the muscles without interleaved relaxation and that can result in death; (5) a reaction can occur if the white muscles of the fish are stimulated to the point of an epileptic seizure, thereby causing morphological trauma.
In normal electrofishing practice, direct current or pulsed direct currents are used because aquatic animals will move, in general, to the anode electrode. In the case of fish, this movement, (e.g. electrotaxis), involves a pseudo swimming reaction. As a fish approaches the anode electrode, it encounters an increasing field strength. At some critical value of field strength, depending upon many physical factors (e.g. water conductivity, fish health, etc) the fish may cease electrotaxis action, then enter a state of narcosis, and then a state of tetanus. Often, the critical state occurs a few feet from the anode electrode or very near it. In either case, the fish almost always drifts near to or may actually touch the anode electrode. The field strength within this zone causing tetanus is very high and a significant flow of electric current through the fish occurs. This electric current is generally believed to stimulate and then overwhelm the neuromuscular system of the fish. It is believed that the overwhelmed neuromuscular system causes the above referenced trauma.